Fix "arc paste" to stop creating pastes with an empty string ("") as the "language"

Summary:
See PHI652. When you echo x | arc paste today, you end up with a Paste object that has the empty string as its "language".

This is normally not valid. Pastes where the language should be autodetected should have the value null, not the empty string.

This behavior likely changed when paste.create got rewritten in terms of paste.edit. Adjust the implementation so it only adds the LANGUAGE transaction if there's an actual language.

Also, fix an issue where you can't use the "delete" key to delete tokens with the empty string as their value.

Test Plan:

• Created a paste with echo x | arc paste, got a paste in autodetect mode instead of with a bogus language value.
• Created a paste with echo x | arc paste --lang rainbow, got a rainbow paste.
• Deleted an empty string token with the keyboard.
• Deleted normal tokens with the keyboard.
• Edited subscribers/etc normally with the keyboard and mouse to make sure I didn't ruin anything.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19437